The invention relates to a liquid-crystalline side chain polyurethane. Such a polyurethane is known from Makromol. Chem., 189 (1988), 771-776 (Tanaka). This article describes a liquid-crystalline polyurethane containing an azobenzene mesogenic group in the side chain attached to the main chain via a flexible spacer. A drawback is that a medium for optical data storage composed of these polyurethanes is not suitable for diazotype copying--a conventional technique for copying these media--on account of absorption at about 400 nanometers. Another drawback to this mesogenic group is its absorption of light in the visible range. Under the influence of light these liquid-crystalline polyurethanes isomerize, which may hamper the long term stability of the written film. The invention has as an object the elimination of these drawbacks.